America is coming up far short in its efforts to counter Islamic State propaganda, and the jihadists' messages often resonate with younger people, the US military's top general warned Monday.

"I think we probably do get a C-minus or D in terms of doing it right now," General Joe Dunford told a national security forum in Washington.

Dunford, whose official title is Chairman of the Joint Chiefs of Staff, said people in the West tend to overlook the power of IS on social media -- even as the jihadists' propaganda captures the imagination of some.

"What's concerning to me is... the narrative that ISIL has is getting traction and we need to take that part of it serious," he said, using an alternative acronym for the IS group.

"We can look at the absurdity of the ideas and immediately be dismissive. It's easy to do but those ideas resonate," he added.

"Amazingly enough they are resonating with young people here in the United States who are either disaffected, dislocated or just not fully integrated in our society."

Dunford, who previously headed the Marine Corps, started his new role at the Pentagon at the end of September.

He appeared at a briefing earlier Monday with President Barack Obama, who voiced fresh determination to destroy the IS group, vowing to kill their leaders and win back territory in the Middle East.

The United States has since August 2014 been leading a coalition bombing the IS group in Iraq and Syria.